A video signal includes at least three levels of voltages; the sync tip voltage level, the black voltage level and the peak white voltage level. For the case of positive modulation schemes, the sync tip voltage level is approximately 10% of the full scale carrier voltage level, the black voltage level is approximately 30% the full scale carrier voltage level, and the peak white voltage level is at 100% the full scale carrier voltage level.
A modulated video signal is received via a demodulation circuit which comprises an intermediate frequency (IF) amplifier, a demodulator and a peak detector for controlling the gain of the IF amplifier. The IF amplifier has its input coupled to receive, via a tuner, the modulated video signal. The demodulator takes the modulated video signal at the output of the IF amplifier and outputs the video signal. This video signal is then sent into a first input of a peak detector wherein the second input of the peak detector is coupled to receive a reference voltage level. The peak detector functions to detect a peak voltage on the video signal and adjust the gain of the IF amplifier to ensure that the peak white voltage level is maintained at a predetermined level.
When a negative modulation scheme is utilized, a relatively fast acting peak detector or gated automatic gain control system can be used to monitor and control the demodulated sync tip level because the sync tip level for negative modulation is at 100% of the full scale carrier level. However, such an approach cannot be used when a positive modulation scheme is utilized because even if the black voltage level, which is at 30% of the full scale carrier level is controlled in a predetermined fashion, a small error in the black voltage level at the transmitter (which is common for systems in France) translates into a substantially large error in the peak white voltage level. As a result, automatic gain control circuits for positive modulation TV signals use the peak detector to detect the peak white voltage level of the video signal. Thus, a second input of the peak detector is coupled to receive a peak white reference voltage level. However, a long time constant must be used with respect to the peak detector because the video signal may only be at a peak white voltage level for a short period of time during the picture. As a result, the AGC system is slow and cannot track signal fluctuations very well. Further, the TV picture may go blank for a substantial number of seconds upon changing channels.
Hence, there exists a need for an improved automatic gain control system for use in positive modulation schemes for video signals.